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Intellectual Merit- Ceratopsians are among the best known of dinosaur clades due to their relative abundance, and the remarkable diversity of cranial synapomorphies such as horns and frills that diagnose more derived members of the clade. Gigantism is a trend in ceratopsian evolution that is accompanied by the evolution of progressively more complex feeding mechanisms and cranial display diversity. Heterochrony, and in particular peramorphosis have been proposed as major influences in ceratopsian evolution to explain the evolution towards large size and late ontogenetic genesis of the display in advanced forms. Likewise, the apparent hierarchical complexity of the feeding apparatus suggests an evolution of character integration through both ontogeny and phylogeny. All of these evolutionary hypotheses require detailed knowledge of both the development and phylogeny of ceratopsian dinosaurs, but so far age estimates have been poorly constrained and non-quantified and rigorous reliance on detailed phylogenies limited. In this study, an innovative approach to developmentally staging fossils based on histological sectioning and Developmental Mass extrapolation is combined with novel tree-based methods toward studying heterochrony and integration using a well- established phylogeny of Ceratopsia. Based on well-sampled growth series of seven -backbone taxa representing the diversity of the clade, and a detailed phylogeny of ceratopsians, we will examine whether there are correlated shifts in developmental timing of synapomorphies (heterochrony) relative to growth strategies, and also if blocks of characters represent integrated modules that fulfill criteria for being phylogenetically and ontogenetically constrained. These quantitative studies will allow us to evaluate previous hypotheses on the role heterochrony and integration have played in ceratopsian evolution. By providing a quantified understanding of when in ontogeny synapomorpies are expressed this research will help to resolve several taxonomic quandaries related to the validity of species based on juvenile holotypes and provide a more accurate understanding of ceratopsian diversity. Broader Impact: This research represents a new integrative exploration of evolutionary processes in a well-known group of dinosaurs. The novel methods and the results of this research promise to be widely applicable to fields as diverse as population biology, developmental biology, macroevolution and evo-devo, and will be disseminated through peer -reviewed journals, presentations at professional meetings and professional web- sites. Two graduate and two undergraduate students will be trained through this project, and aspects of the research will be incorporated into university courses taught by both PIs. Both PIs are involved with developing exhibits for natural history museum, and parts of this research will be incorporated into two ongoing exhibit projects, and associated educational web-sites and programming. Given the media interest in dinosaur research, many opportunities to disseminate results of this work to the public will no doubt arise.